fftw3: Fortran-interface routines
8.1 Fortran-interface routines
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Nearly all of the FFTW functions have Fortran-callable equivalents. The
name of the legacy Fortran routine is the same as that of the
corresponding C routine, but with the 'fftw_' prefix replaced by
'dfftw_'.(1) The single and long-double precision versions use 'sfftw_'
and 'lfftw_', respectively, instead of 'fftwf_' and 'fftwl_'; quadruple
precision ('real*16') is available on some systems as 'fftwq_' (
Precision). (Note that 'long double' on x86 hardware is usually at
most 80-bit extended precision, _not_ quadruple precision.)
For the most part, all of the arguments to the functions are the
same, with the following exceptions:
* 'plan' variables (what would be of type 'fftw_plan' in C), must be
declared as a type that is at least as big as a pointer (address)
on your machine. We recommend using 'integer*8' everywhere, since
this should always be big enough.
* Any function that returns a value (e.g. 'fftw_plan_dft') is
converted into a _subroutine_. The return value is converted into
an additional _first_ parameter of this subroutine.(2)
* The Fortran routines expect multi-dimensional arrays to be in
_column-major_ order, which is the ordinary format of Fortran
arrays (Multi-dimensional Array Format). They do this
transparently and costlessly simply by reversing the order of the
dimensions passed to FFTW, but this has one important consequence
for multi-dimensional real-complex transforms, discussed below.
* Wisdom import and export is somewhat more tricky because one cannot
easily pass files or strings between C and Fortran; see
Wisdom of Fortran?.
* Legacy Fortran cannot use the 'fftw_malloc' dynamic-allocation
routine. If you want to exploit the SIMD FFTW (SIMD
alignment and fftw_malloc), you'll need to figure out some other
way to ensure that your arrays are at least 16-byte aligned.
* Since Fortran 77 does not have data structures, the 'fftw_iodim'
structure from the guru interface (Guru vector and transform
sizes) must be split into separate arguments. In particular, any
'fftw_iodim' array arguments in the C guru interface become three
integer array arguments ('n', 'is', and 'os') in the Fortran guru
interface, all of whose lengths should be equal to the
corresponding 'rank' argument.
* The guru planner interface in Fortran does _not_ do any automatic
translation between column-major and row-major; you are responsible
for setting the strides etcetera to correspond to your Fortran
arrays. However, as a slight bug that we are preserving for
backwards compatibility, the 'plan_guru_r2r' in Fortran _does_
reverse the order of its 'kind' array parameter, so the 'kind'
array of that routine should be in the reverse of the order of the
iodim arrays (see above).
In general, you should take care to use Fortran data types that
correspond to (i.e. are the same size as) the C types used by FFTW. In
practice, this correspondence is usually straightforward (i.e.
'integer' corresponds to 'int', 'real' corresponds to 'float',
etcetera). The native Fortran double/single-precision complex type
should be compatible with 'fftw_complex'/'fftwf_complex'. Such simple
correspondences are assumed in the examples below.
---------- Footnotes ----------
(1) Technically, Fortran 77 identifiers are not allowed to have more
than 6 characters, nor may they contain underscores. Any compiler that
enforces this limitation doesn't deserve to link to FFTW.
(2) The reason for this is that some Fortran implementations seem to
have trouble with C function return values, and vice versa.
Info Catalog
fftw3: Calling FFTW from Legacy Fortran
fftw3: Calling FFTW from Legacy Fortran
fftw3: FFTW Constants in Fortran